While Android smartphones have surpassed the iPhone in sales a long time ago, and last year, they surpassed the whole iOS install base, too (iOS had a headstart there), the Play Store is the last major milestone for Google to achieve, and beat Apple on it, too.
There are multiple reasons for why the Play Store has remained behind the Apple app store. One of them has to do with Google themselves, which until 2011 or so didn't put a whole lot of effort into making a good app store for both users and developers. They just let everything grow organically. And only when they renamed the store from Android Market to Play Store, they really started caring about the success of the store, and even started promoting it in ads with the new devices.
Another reason is Apple's own success with the iPhone, and later with the iPad, which maintained the Apple platform in the minds of developers. Just as developers were getting ready to port all of their apps to Android, Apple launched the iPad in 2010, and instead of moving to Android, most of them redoubled their efforts on iOS, and tried to make iPad apps instead. This delayed an avalanche of apps coming to Android back then, and the success of iPad also took developers away from even optimizing their Android apps for Android tablets. All of this meant fewer quality apps arrived on Android, and therefore less aggregate revenue to be made on the Play Store.
But now things are looking up, and while the iOS store revenues have kept climbing at a steady rate of 20% or so, it seems the Play Store revenue is climbing much faster, as many developers start focusing not just on regular Android apps but also on tablet apps. For example the Play store saw an increase in revenue from December to January of 700%. A bit part of it could be explained by people buying more apps on Christmas days, but the trend is clear, and if this keeps up it shouldn't be more than a year until the Play Store surpasses the iTunes App Store in revenues.